The grain oriented silicon steel sheets are mainly used as a core for transformers and other electrical machineries, and are required to be excellent in magnetic properties, particularly magnetization property and iron loss property.
The magnetic properties of the grain oriented silicon steel sheet are strongly affected by not only the sheet quality but also its surface properties. For example, the smaller the surface roughness, the better the magnetic properties as disclosed in Japanese Patent laid open No. 59-38326.
Therefore, a rolling treatment rendering the surface roughness of the steel sheet into a center-line average roughness Ra of not more than 0.4 .mu.m, which is called bright finishing, is adopted at the cold rolling step.
As the surface roughness or specific surface area increases, the surface enriching amount of MnS or MnSe acting as an agent inhibiting normal growth of crystal grain (inhibitor) increases to weaken the inhibitor effect inside the steel sheet in the secondary recrystallization annealing step, and consequently the growth of recrystallized grains is insufficient. Further, when the surface of the finally cold rolled steel sheet becomes rough, not only the unevenness of the surface of the product sheet is large, but also the insulating film formed on the sheet surface is thick and uneven, so that when the product sheet is magnetized, the movement of magnetic domains is obstructed.
Furthermore, when the steel sheet contains 2.5.about.4.0 wt % (hereinafter shown by % simply) of Si as in the grain oriented silicon steel sheet, it is very brittle and is apt to be broken as compared with ordinary steel, and also the deformation resistance is very high, so that the cold rolling is generally carried out at a low speed of not more than about 700 mpm using a reverse mill such as sendzimir mill having a small roll diameter (roll diameter: about 80 mm). Therefore, the rolling efficiency is low and the productivity is poor.
The surface roughening due to oxidation scale will be described below.
The hot rolled sheet as a base sheet for silicon steel sheet is subjected to two or more-times cold rolling through an intermediate annealing up to a sheet thickness for final product. In the intermediate annealing, oxidation scale is produced at a thickness of about 0.2.about.3 .mu.m on the surface of the steel sheet. This oxidation scale consists mainly of silicon dioxide (SiO.sub.2) and is very hard and acts upon the rolling roll as in abrasive grains to wear the roll surface, which is transferred to a cold rolled sheet to roughen the surface of the steel sheet.
In this connection, the applicants have previously proposed a method wherein the silicon steel sheet adhered at its surface with a scale layer after intermediate annealing is rolled in a cold tandem rolling machine line while descaling with the use of a descaling device particularly arranged between a first stand and a second stand in Japanese Patent laid open No. 63-119925 as a method for reducing the wearing of the rolling roll.
In the above method, however, there are still the following problems:
1 The surface of the rolling roll in the first stand is roughened by the scale to shorten the life of the roll, so that roll change should frequently be made.
2 The broken scale adheres to the surface of the roll, which is transferred to the surface of the steel sheet, resulting in the occurrence of surface defects, and hence the quality of the steel sheet is lowered.
Next, surface roughening due to the rolling lubricant will be described.
FIG. 2 is a side view diagrammatically showing clipping the steel sheet by the rolling roll. For simplification of the explanation, it is assumed that the surfaces of a roll 2 and a steel sheet 1 before rolling are smooth. In the rolling, a rolling oil is normally used for mitigating rolling load, but this example is a case of using no rolling oil. In this figure, the contact between the roll 2 and the steel sheet 1 starts from a point A. At this point A, the steel sheet 1 begins to cause plastic deformation. The steel sheet 1 and the roll 2 metallically contact each other because there is no rolling oil. Therefore, the rolling load considerably increases, and consequently rolling may be impossible.
On the contrary, FIG. 3 shows diagrammatically a steel sheet clipped into the rolling roll 2 using rolling oil. When the viscosity of the rolling oil is large and particularly the diameter of the rolling roll or the rolling speed in the tandem mill is large, the pressure of the rolling oil 3 produced in the wedge passway at the clipped portion of the rolling roll 2 reaches to the yield stress of the steel sheet 1 at a point B on the way to the point A being the contact point between the rolling roll 2 and the steel sheet 1 shown in FIG. 2.
Therefore, the steel sheet 1 is subjected to plastic deformation, but this is a free deformation in the rolling oil 3, so that unevenness is caused in the sheet. Furthermore, the rolling oil 3 enters in the clipped region, and the deformation increases to increase the unevenness. When the unevenness becomes larger than the thickness of the oil film, the oil film is broken to start contacting between the roll and the steel sheet at a point C. The convex portion of the steel sheet 1 contacted with the rolling roll 2 is flattened by the rolling roll 2, but the concave portion is not flattened because the rolling oil 3 is filled in the concave portion, and hence the concave portion is retained as it is to make the surface of the steel sheet rough.
An example of the uneven state is shown in FIG. 4. This shows a so-called three-dimensional profile obtained by measuring height direction (Z) of the unevenness while moving a probe in lengthwise direction (X) on the surface of the steel sheet by means of a surface roughness meter, further moving the probe in widthwise direction (Y) by a given position and repeating the same measurement.
The concave portion of the steel sheet through the rolling oil can be made small by reducing the viscosity of the rolling oil, which never arrives at the level of a bright sheet.